feat: implement class method definition and instance creation in compiler and VM

Add method parsing and compilation in `statement()` for class bodies, including symbol interning and block constant emission. Introduce `addConstant()` helper in compiler.c. Extend VM with `CODE_METHOD` opcode, `ObjInstance` type, `makeInstance()` factory, metaclass support in `makeClass()`, and `getSymbolName()` for symbol table lookup. Update vm.h with new type enum, struct definitions, and function declarations.

example/hello.wren

@@ -1,6 +1,7 @@
-var a = {
-  var b = 234
-  3
-  b
+class Foo {
+  bar {
+    123
+  }
 }
-a.call
+var a = Foo.new
+a.bar